Multilayer ceramic capacitors are an interesting ceramic electronic component application for the present invention. A multilayer ceramic capacitor includes a capacitor body as a ceramic electronic component body. Each end face of the capacitor body has a terminal electrode.
In a multilayer ceramic capacitor mounted on a circuit board, a stress applied to the capacitor body by the circuit board due to a thermal or mechanical factor may cause mechanical damage such as a crack to the capacitor body. Thus, to relieve such a stress and avoid mechanical damage, a multilayer ceramic capacitor having a structure in which a metal terminal is attached to a terminal electrode has been proposed.
In such a multilayer ceramic capacitor having a metal terminal, solder is generally used as a binder for joining a terminal electrode to the metal terminal, as described in Japanese Patent No. 3376971 (Patent Document 1).
However, when a multilayer ceramic capacitor is mounted on a circuit board, the capacitor body may fall off a metal terminal during a solder reflow process because solder has a relatively low melting point. Furthermore, an intermetallic compound, such as Cu3Sn or Ag3Sn, may be formed by the reaction between the solder and a metal terminal. Because of the production of the intermetallic compound or difference in the coefficient of linear expansion between a metal terminal and solder, the thermal shock may cause a crack to form at the interface between the solder and the metal terminal, thus reducing the mechanical reliability.
In view of the circumstances, Japanese Unexamined Patent Application Publications Nos. 2002-231569 (Patent Document 2) and 2004-47671 (Patent Document 3) proposes that the terminal electrode and the metal terminal be joined using a Ag—Cu alloy to increase the heat resistance. Although the Ag—Cu alloy joint between the terminal electrode and the metal terminal has a high heat resistance, the bonding strength of the joint is not necessarily good.
The present inventor has investigated the cause of the low bonding strength and found that, in general, a plated layer or a paste layer is substantially lost at relatively high temperature, such as the temperature required to form the Ag—Cu alloy, that is, the eutectic temperature of the Ag—Cu alloy of 779° C., because Ag or Cu forming the plated layer or the paste layer diffuses into an adjacent layer and forms, Kirkendall voids therein. Although Patent Document 2 does not describe a specific alloying temperature, Patent Document 3 describes an alloying temperature of 800° C.
The diffusion of Ag or Cu is most likely to occur in the plated layer and is likely to occur in a bonding paste layer and a terminal electrode or a metal terminal in this order. For example, when a joint is composed of “Cu terminal electrode—Cu paste bonding layer—Ag plated layer—Cu metal terminal” in this order, the Ag plated layer first diffuses into the Cu paste bonding layer and forms Kirkendall voids therein. Thus, the Ag plated layer is lost.
In an example of Patent Document 2, a drop test is performed under a load to determine the bonding strength. According to Patent Document 2, the capacitor element does not fail under a load of 20 g. However, a load of 20 g is a very small load. The resistance to this load therefore does not necessarily mean that the bonding strength is good. The bonding strength is considered good when the bonding strength is larger than 20 g by at least one order of magnitude. The reason that the bonding strength in Patent Document 2 is not so high may be that heat treatment in alloying causes a Ag film in the metal terminal to diffuse into a Cu film in the terminal electrode, thus eliminating the Ag film.
Patent Document 1: Japanese Patent No. 3376971
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-231569
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-47671